Environmental Engineering Reference
In-Depth Information
are partially crystalline, this transition can only occur in the amorphous
fraction, leaving the crystallites virtually untouched. As partial crystallinity,
cross-linking of the polymer chains and tacticity of polymer all affect the
freedom of the chain segments to move in response to heating, these also
increase the value of Tg. Cross-linking, for instance, restricts such motion
increasing the Tg of the polymer, while the higher concentration of chain
ends (shorter average chain lengths) with decreasing average molecular
weight or the presence of certain low-molecular-weight compounds
(plasticizers) increases free volume, plasticizing the polymer and reducing
the Tg. Below its Tg, the polymer is either a glass or a glass embedded with
crystallites; these are hard and brittle (low impact strength) solids with high
strength and modulus. While above Tg, the same polymer will turn into
rubbery and tough materials with good elasticity.
We generally observe and use polymers at ambient temperatures; our
perception of their being glassy or rubbery will depend on if they happen
to be above or below their Tg at the ambient temperature of observation.
For instance, PS has a Tg of approximately 100°C and is a hard “plastic”
material at ambient temperature
T
°C (
T
< Tg). The Tg of natural rubber
cis
-polyisoprene is −70°C (and
T
> Tg), which makes the polymer have a
soft rubbery feel at ambient temperature, which is ~90°C above its Tg. If the
temperature of a rubber is decreased below its Tg (e.g., immerse a rubber
ball in liquid nitrogen), it will behave like a glass and even shatter on being
dropped on the floor. Similarly, PS, a hard plastic, heated above its Tg of
100°C will yield a soft rubbery material. Values of Tg for common plastics
given in
Table 3.1
illustrate that these polymers have values well above the
ambient temperatures. These values depend on average molecular weight,
polydispersity, and the heating rate used to determine it.
